The present invention relates to a method of applying a liquid such as a photographic photosensitive liquid, a magnetic liquid and a surface protective liquid to a flexible band-like carrier (which is hereinafter referred to as a web) made of a plastic film, paper, a metal leaf or the like. More particularly, the invention relates to a method of applying a liquid to the surface of a flexible band-like carrier while pushing the outlet portion of an application head toward the surface of the carrier being continuously conveyed.
Various methods have been proposed and practiced, in each of which a liquid is applied to a web being continuously conveyed. One of the methods is practiced with an extrusion-type application device which is used in various fields, as mentioned in Japan Patent Applications (OPI) Nos. 138036/75 and 84771/80 (the term "OPI" as used herein means an "unexamined published application") and Japanese Patent No. 7306/79. However, the extrusion-type application device has a range of proper liquid application which is very narrow. Particularly, when the liquid application speed is greater than or equal to 100 to 150 m/min., it is very difficult for the device to apply the liquid to the web stably and properly so that the applied liquid makes a layer having a thickness of 20.mu. or less on the web. This is a problem.
As a result of studies, the present inventor found out that this problem occurs because the quantity of air resulting from the web being conveyed and impacting the device's application head increases sharply when the application speed is increased to 100 to 150 m/min. or more. The air impacting the head makes it difficult to apply the liquid to the web in a layer having a uniform thickness.
To solve this problem, a method of application was disclosed in Japan Patent Application (OPI) No. 205561/83. In this method, a liquid which is substantially the same as an applied liquid, which is applied to the surface of a web, is supplied from an upstream (relative to the conveyance direction of the web) slot to the web surface, and the applied liquid is continuously supplied by a prescribed quantity from a downstream slot to the web surface simultaneously with the liquid from the upstream slot. Thus, the upstream slot liquid is between the web surface and the applied liquid, thereby to prevent air from being trapped between the web surface and the applied liquid, and thus performing the application to make a layer having a uniformly flat surface.
Furthermore, as a means to solve the above-mentioned problem, an application device was disclosed in Japan Patent Application (OPI) No. 238179/85. As shown in FIG. 3, the extrusion-type application head 22 of the device has a doctor edge portion having a curved surface 24 so that a pressurized liquid accumulation 27 is formed on the surface when applying a liquid to a web 6 to appropriately control the pressure of the liquid at the outlet portion of a slot 26. This prevents air caused by the movement speed of the web from being trapped between the applied liquid and the web. Thus, the liquid can be applied to the web 6 at a high speed of 300 m/min. by the device so as to form a layer 28 which is not streaked and which has a uniform thickness.
Another method of application was disclosed in Japan Patent Application (OPI) No. 139929/86. In this method, a solvent which is substantially similar to a liquid which is applied to the web surface, is applied to the surface before the liquid is applied, so that the inner surface of the layer of the liquid jetted from an outlet portion of a slot is separated from ambient air by the solvent when the liquid is subsequently applied. Thus, the liquid can be applied to a web moving at a high speed so as to make a thin layer thereon.
However, the above-mentioned method disclosed in Japan Patent Application (OPI) No. 205561/83 has a problem in that when the application speed of the applied liquid is increased dramatically, air is likely to impact between the web surface and the liquid supplied from the upstream slot, so as to vibrate the liquid to affect the applied liquid. Thus, the thickness of the layer of the applied liquid on the web will be non-uniform. Furthermore, the application device disclosed in Japan Patent Application (OPI) No. 238179/85 and described above has a problem in that when the application speed of the liquid is increased to form a layer of smaller thickness, air is trapped in the layer to make it impossible to stably and properly perform the application.
Although the inner surface layer of the applied liquid is separated from the air by the solvent applied to the web prior to liquid application to the web, in the application method disclosed in Japan Patent Application (OPI) No. 139929/86, a problem results in that when the application speed of the liquid is increased and the thickness of the solvent layer is reduced, it is difficult to stably separate the inner surface of the layer of the applied liquid from the air by the solvent, and air is likely to be trapped between the liquid layer and the solvent layer at the back edge portion of an extrusion-type application head.
In view of the foregoing, it is technically difficult to apply a liquid to a web having a conveyance speed of 300 m/min. or more in each of these conventional systems and methods.